It is desirable in many circumstances to allow building entry doors, windows, cupboards etc. to be opened only by those who have been authorised to do so. One way of achieving this is to use a mechanical locking mechanism which requires a predetermined code to be entered before it can be unlocked and mechanical combination door locks are widely used for this purpose.
Such locks are typically provided with a series of alphanumeric buttons (often 4, 6, 8, 10 or 12 buttons) and a knob or handle which can be grasped by the user. In operation, an authorised user dials the correct passcode into the mechanism, then turns the knob or handle in order to unlock the mechanism. The passcode on such locks can typically be changed to suit a user's preference by dismantling a portion of the lock (normally from the rear) and switching the orientation of certain locking pins provided in the body of the lock; however, for security reasons this normally requires that the lock is first opened to gain access to the rear/inside of the door before then removing the lock from its mount on the door. This can be relatively time consuming, is often inconvenient, and requires a degree of skill and care in order to correctly change the code and to correctly re-mount the lock back on the door.